Rheumatoid arthritis (hereinafter referred to as “RA”) is a systemic inflammatory disorder of unknown etiology, often characterized by frequent occurrence of erosive arthritis and also involving disorders in multiple organs. RA chronically progresses by repeating a cycle of remission and exacerbation, and, if untreated, causes destruction or deformation of joints, which eventually leads to functional disabilities in motor organs and sometimes death. Therefore, RA causes huge physical and psychological pain throughout the life of a patient.
RA develops in a variety of ways, and the guidelines proposed by the American College of Rheumatology are widely used for the diagnosis. However, RA usually develops gradually over the course of several weeks to several months, and the percent positive for the rheumatoid factor, which is used as an objective measure of RA diagnosis according to the foregoing guidelines, is only about 33% within 3 months and does not go beyond 88% even within 12 months or longer (see Non-Patent Document 1: Treatment, Vol. 73, No. 3, pp. 23-27, 1991). That is, there is inaccuracy in RA diagnosis. In view of this, there have been attempts to diagnosis RA by detecting RA-associated anti-IgM antibody that reacts with a recombinant antigen in the serum of a patient (see Patent Document 1: Japanese Laid-Open Patent Publication No. 10-513257 (published on Dec. 15, 1998)).
Further, the treatment of RA generally follows different paths depending upon the progress of the pathology. In the early stage of RA where the diagnosis is generally premature, the treatment involves administration of non-steroidal anti-inflammatory drugs (NSAID). This is followed by additional administration of disease-modifying antirheumatic drugs (DMARD), once the diagnosis becomes definitive. Specifically, in the initial stage of RA development where definitive diagnosis cannot be made easily, the treatment begins with the administration of NSAID, and while carefully monitoring the progress, an evaluation is made to distinguish whether the patient is actually suffering from RA or other rheumatoid disorders including collagenosis. If the symptoms progress, steroid drugs may be administered, and the drug therapy for relieving pain is performed in combination with physical therapy and device therapy, which are intended to maintain and restore the joint functions. Surgery may be performed if daily living activities were disabled by damaged joints.
As described in Patent Document 2 (International Publication WO98/51791 (published on Nov. 19, 1998)), the inventors of the present invention have performed linkage analysis on RA patients and their kin group using micro satellite markers, and specified 3 loci for RA genes. Specifically, the following affected genes have been identified.                (1) RA gene located on human chromosome 1 within ±1 centiMorgan of the DNA sequence that hybridizes with micro satellite marker D1S214 and/or D1S253        (2) RA gene located on human chromosome 8 within ±1 centiMorgan of the DNA sequence that hybridizes with micro satellite marker D8S556        (3) RA gene located on human X chromosome within ±1 centiMorgan of the DNA sequence that hybridizes with micro satellite marker DXS1001, DXS1047, DXS1205, DXS1227, and/or DXS1232        
Further, as described in Non-Patent Document 2 (Rheumatism, Vol. 39, No. 2, pp. 444-445, 1999), the inventors of the present invention, from the RA-associated genes of the prior invention, has identified death receptor 3 (“DR3” or “DR3 gene”) as a candidate for the RA-associated gene for markers D1S214 and D1S253 as set out in (1) above. Concerning DR3, the inventors have confirmed restriction fragment length polymorphism between healthy subjects and RA patients, and suggested the possibility that DR3 might be a gene associated with RA.
In Patent Document 3 (International Publication WO02/34912 (published on May 2, 2002)), the inventors of the present invention have suggested that genomic mutation in DR3 gene might be related to onset of RA (chronic RA).
In higher eukaryotes, methylation is known to occur in portions of genomic DNA sequence with 5′-CG-3′ (hereinafter, “CpG” or “CpG sequence”), whereby the cytosine (C) on the 5′ end side of guanosine (G) is methylated. The methylation in CpG is believed to have influence on the gene expression, particularly when CpG-rich regions exist within the promoter region of the gene.
As a rule, many genes on the chromosomes are protected from methylation. However, gene transcription is suppressed if, for some reason, methylation occurs in CpG-rich regions in the promoter region. From this, it can be speculated that the effect of CpG methylation on gene expression can impede proper gene activity and cause disease.
However, the methylation state of CpG needs to be studied further for DR3, a possible candidate for RA-associated gene.